chapter 23: the evolution of populations

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Chapter 23: The Evolution of Populations. Chapter 23 Assignment. Define the terms population , species , gene pool , relative fitness , and neutral variation List the five conditions of Hardy-Weinberg equilibrium. Hardy – Weinberg Equilibrium Problems (to be distributed later ). - PowerPoint PPT Presentation

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Page 1: Chapter 23: The Evolution of Populations

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Chapter 23: The Evolution of Populations

Page 2: Chapter 23: The Evolution of Populations

Chapter 23 Assignment

1. Define the terms population, species, gene pool, relative fitness, and neutral variation

2. List the five conditions of Hardy-Weinberg equilibrium.

3. Hardy – Weinberg Equilibrium Problems (to be distributed later)

Page 3: Chapter 23: The Evolution of Populations

Overview: The Smallest Unit of Evolution

One misconception is that organisms evolve, in the Darwinian sense, during their lifetimes

Natural selection acts on individuals, but only populations evolve

Genetic variations in populations contribute to evolution Microevolution is a change in allele frequencies in a

population over generations

Page 4: Chapter 23: The Evolution of Populations

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Concept 23.1: Mutation and sexual reproduction produce

the genetic variation that makes evolution possible

Page 5: Chapter 23: The Evolution of Populations

Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

Page 6: Chapter 23: The Evolution of Populations

Genetic Variation

Variation in individual genotype leads to variation in individual phenotype

Not all phenotypic variation is heritable Natural selection can only act on

variation with a genetic component

Page 7: Chapter 23: The Evolution of Populations

Fig. 23-2

(a)

(b)

Nonheritable variation: these caterpillars display phenotypes based on their diets, not their genes.

Page 8: Chapter 23: The Evolution of Populations

Variation Within a Population

Both discrete and quantitative characters contribute to variation within a population

Discrete characters can be classified on an either-or basis

Quantitative characters vary along a continuum within a population

Page 9: Chapter 23: The Evolution of Populations

Average Heterozygosity

Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels

Average heterozygosity measures the average percent of loci that are heterozygous in a population

Page 10: Chapter 23: The Evolution of Populations

Variation Between Populations

Geographic variation: differences between gene pools of separate populations or population subgroups

Page 11: Chapter 23: The Evolution of Populations

Fig. 23-3

13.17

19 XX10.16

9.128.11

1 2.4

3.14 5.18 6 7.15

9.10

1 2.19

11.12

13.17

15.18

3.8 4.16 5.14 6.7

XX

Page 12: Chapter 23: The Evolution of Populations

• Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis• Examples: frequency of an allele present

in the population`

Page 13: Chapter 23: The Evolution of Populations

How do new alleles arise?

Mutations: Gene (point mutations) Chromosomal

Sexual reproduction

Page 14: Chapter 23: The Evolution of Populations

Mutation

Mutations are changes in the nucleotide sequence of DNA

Mutations cause new genes and alleles to arise

Only mutations in cells that produce gametes can be passed to offspring

Page 15: Chapter 23: The Evolution of Populations

Point Mutations

A point mutation is a change in one base in a gene Point mutations can vary in severity:

Noncoding regions – point mutation has no effect on gene expression

Can be more severe – sickle-cell disease

Rarely do mutations increase the organism’s fitness – most mutations have a negative effect on the organism

Page 16: Chapter 23: The Evolution of Populations

Mutations That Alter Gene Number or Sequence

Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful

Duplication of large chromosome segments is usually harmful Duplication of small pieces of DNA is sometimes less

harmful and increases the genome size Duplicated genes can take on new functions by further

mutation

Page 17: Chapter 23: The Evolution of Populations

Mutation Rates

Mutation rates are low in animals and plants Why?

The average is about one mutation in every 100,000 genes per generation

Mutations rates are often lower in prokaryotes and higher in viruses Why are mutations higher in viruses?

Page 18: Chapter 23: The Evolution of Populations

Sexual Reproduction

3 mechanisms: Crossing over Independent assortment Fertilization

Shuffle existing alleles into new combinations In organisms that reproduce sexually, recombination of

alleles is more important than mutation in producing the genetic differences that make adaptation possible

Page 19: Chapter 23: The Evolution of Populations

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Concept 23.2: The Hardy-Weinberg equation can be

used to test whether a population is evolving

Page 20: Chapter 23: The Evolution of Populations

Gene Pools and Allele Frequencies

• A population is a localized group of individuals capable of interbreeding and producing fertile offspring

A gene pool consists of all the alleles for all loci in a population

Page 21: Chapter 23: The Evolution of Populations

Fig. 23-5 Porcupine herd

Porcupineherd range

Beaufort Sea

NORTHWEST

TERRITORIES

MAPAREA

ALA S

KA

CAN

ADA

Fortymileherd range

Fortymile herd

ALAS

KAYU

KON

Page 22: Chapter 23: The Evolution of Populations

Hardy-Weinberg Principle

Determine if a population is evolving or not Calculate allele frequencies of hypothetical non-

evolving populations and compare to actual frequencies of sample population Allele frequencies should remain constant from one

generation to the next if there is no evolution

Page 23: Chapter 23: The Evolution of Populations

The frequency of an allele in a population can be calculated For diploid organisms, the total number of alleles at a locus is

the total number of individuals x 2 The total number of dominant alleles at a locus is 2 alleles for

each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

Page 24: Chapter 23: The Evolution of Populations

By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies

The frequency of all alleles in a population will add up to 1 For example, p + q = 1

Page 25: Chapter 23: The Evolution of Populations

Fig. 23-6

Frequencies of alleles

Alleles in the population

Gametes producedEach

egg:Each sperm:

80%chance

80%chance

20%chance

20%chance

q = frequency of

p = frequency ofCR allele = 0.8

CW allele = 0.2

Page 26: Chapter 23: The Evolution of Populations

Hardy-Weinberg equilibrium describes the constant frequency of alleles in such a gene pool

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then p2 + 2pq + q2 = 1 where p2 and q2 represent the frequencies of the

homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Page 27: Chapter 23: The Evolution of Populations

Hardy-Weinberg Equilibrium Equation

p2 + 2pq + q2 = 1 Where p2 and q2 represent the

frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

Page 28: Chapter 23: The Evolution of Populations

Fig. 23-7-1

SpermCR

(80%)

CW

(20 % )

80% CR ( p = 0.8)

CW (20%)

20% CW (q = 0.2)

16% ( pq) CRCW

4% (q2) CW CW

CR

(80% )

64% ( p2) CRCR

16% (qp) CRCW

Eggs

Page 29: Chapter 23: The Evolution of Populations

Fig. 23-7-2

Gametes of this generation:

64% CRCR, 32% CRCW, and 4% CWCW

64% CR   +    16% CR    =   80% CR = 0.8 = p

4% CW     +    16% CW   =   20% CW = 0.2 = q

Page 30: Chapter 23: The Evolution of Populations

Fig. 23-7-3

Gametes of this generation:

64% CRCR, 32% CRCW, and 4% CWCW

64% CR   +    16% CR    =   80% CR = 0.8 = p

4% CW     +    16% CW   =   20% CW = 0.2 = q

64% CRCR, 32% CRCW, and 4% CWCW plants

Genotypes in the next generation:

Page 31: Chapter 23: The Evolution of Populations

Conditions for Hardy-Weinberg Equilibrium

The Hardy-Weinberg theorem describes a hypothetical population

In real populations, allele and genotype frequencies do change over time

Page 32: Chapter 23: The Evolution of Populations

The five conditions for nonevolving populations are rarely met in nature: No mutations Random mating No natural selection Extremely large population size No gene flow

Page 33: Chapter 23: The Evolution of Populations

Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

Page 34: Chapter 23: The Evolution of Populations

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Hardy-Weinberg Practice Problems and Homework

Page 35: Chapter 23: The Evolution of Populations

Practice Problem 1

1. You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the following:• The frequency of the "aa" genotype.• The frequency of the "a" allele.• The frequency of the "A" allele.• The frequencies of the genotypes "AA" and

"Aa.“• The frequencies of the two possible phenotypes

if "A" is completely dominant over "a."

Page 36: Chapter 23: The Evolution of Populations

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Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele

frequencies in a population

Page 37: Chapter 23: The Evolution of Populations

Three major factors alter allele frequencies and bring about most evolutionary change: Natural selection Genetic drift Gene flow

Page 38: Chapter 23: The Evolution of Populations

Natural Selection

Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

Page 39: Chapter 23: The Evolution of Populations

Genetic Drift

The smaller a sample, the greater the chance of deviation from a predicted result

Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the next

Genetic drift tends to reduce genetic variation through losses of alleles

2 examples: Founder Effect, Bottleneck Effect

Page 40: Chapter 23: The Evolution of Populations

Fig. 23-8-1

Generation 1p (frequency of CR) =

0.7q (frequency of CW ) =

0.3

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

Page 41: Chapter 23: The Evolution of Populations

Fig. 23-8-2

Generation 1p (frequency of CR) =

0.7q (frequency of CW ) =

0.3

Generation 2 p =

0.5q = 0.5

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

Page 42: Chapter 23: The Evolution of Populations

Fig. 23-8-3

Generation 1

CW CW

CR CR

CR CW

CR CR

CR CR

CR CR

CR CR

CR CW

CR CW

CR CW

p (frequency of CR) = 0.7q (frequency of CW

) = 0.3

Generation 2

CR CWCR CW

CR CW

CR CW

CW CW

CW CW

CW CW

CR CR

CR CR

CR CR

p = 0.5q = 0.5

Generation 3 p =

1.0q = 0.0

CR CR

CR CR

CR CR

CR CR

CR CR

CR CR CR CR

CR CR

CR CR CR CR

Page 43: Chapter 23: The Evolution of Populations

The Founder Effect

The founder effect occurs when a few individuals become isolated from a larger population

Allele frequencies in the small founder population can be different from those in the larger parent population

Page 44: Chapter 23: The Evolution of Populations

The Bottleneck Effect

The bottleneck effect is a sudden reduction in population size due to a change in the environment

The resulting gene pool may no longer be reflective of the original population’s gene pool

If the population remains small, it may be further affected by genetic drift

Page 45: Chapter 23: The Evolution of Populations

Fig. 23-9

Originalpopulatio

n

Bottlenecking

event

Survivingpopulatio

n

Page 46: Chapter 23: The Evolution of Populations

Effects of Genetic Drift: A Summary

1. Genetic drift is significant in small populations

2. Genetic drift causes allele frequencies to change at random

3. Genetic drift can lead to a loss of genetic variation within populations

4. Genetic drift can cause harmful alleles to become fixed

Page 47: Chapter 23: The Evolution of Populations

Gene Flow

Gene flow consists of the movement of alleles among populations

Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)

Gene flow tends to reduce differences between populations over time

Gene flow is more likely than mutation to alter allele frequencies directly

Page 48: Chapter 23: The Evolution of Populations

Gene flow can decrease the fitness of a population

In bent grass, alleles for copper tolerance are beneficial in populations near copper mines, but harmful to populations in other soils

Windblown pollen moves these alleles between populations

The movement of unfavorable alleles into a population results in a decrease in fit between organism and environment

Page 49: Chapter 23: The Evolution of Populations

Fig. 23-12

NON-MINESOIL

MINESOIL

NON-MINESOIL

Prevailing wind direction

Inde

x of

cop

per

tole

ranc

e

Distance from mine edge (meters)

70

60

50

40

30

20

10

020 0 20 0 20 40 60 80 10

0120

140

160

Page 50: Chapter 23: The Evolution of Populations

Gene flow can increase the fitness of a population

Insecticides have been used to target mosquitoes that carry West Nile virus and malaria

Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes

The flow of insecticide resistance alleles into a population can cause an increase in fitness

Page 51: Chapter 23: The Evolution of Populations

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Concept 23.4: Natural selection is the only mechanism that

consistently causes adaptive evolution

Page 52: Chapter 23: The Evolution of Populations

Only natural selection consistently results in adaptive evolution

Page 53: Chapter 23: The Evolution of Populations

A Closer Look at Natural Selection

Natural selection brings about adaptive evolution by acting on an organism’s phenotype

Page 54: Chapter 23: The Evolution of Populations

Relative Fitness

The phrases “struggle for existence” and “survival of the fittest” are misleading as they imply direct competition among individuals

Reproductive success is generally more subtle and depends on many factors

Page 55: Chapter 23: The Evolution of Populations

Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals

Selection favors certain genotypes by acting on the phenotypes of certain organisms

Page 56: Chapter 23: The Evolution of Populations

Directional, Disruptive, and Stabilizing Selection

Three modes of selection: Directional selection favors individuals at one end of the

phenotypic range Disruptive selection favors individuals at both extremes

of the phenotypic range Stabilizing selection favors intermediate variants and

acts against extreme phenotypes

Page 57: Chapter 23: The Evolution of Populations

Fig. 23-13a

Original population

(a) Directional selection

Phenotypes (fur color)

Freq

uenc

y of

in

divi

dual

s

Original population

Evolved population

Page 58: Chapter 23: The Evolution of Populations

Fig. 23-13b

Original population

(b) Disruptive selection

Phenotypes (fur color)

Freq

uenc

y of

in

divi

dual

s

Evolved population

Page 59: Chapter 23: The Evolution of Populations

Fig. 23-13c

Original population

(c) Stabilizing selection

Phenotypes (fur color)

Freq

uenc

y of

in

divi

dual

s

Evolved population

Page 60: Chapter 23: The Evolution of Populations

The Key Role of Natural Selection in Adaptive

Evolution

Natural selection increases the frequencies of alleles that enhance survival and reproduction

Adaptive evolution occurs as the match between an organism and its environment increases

Page 61: Chapter 23: The Evolution of Populations

Fig. 23-14

(a) Color-changing ability in cuttlefish

(b) Movable jaw bones in snakes

Movable bones

Page 62: Chapter 23: The Evolution of Populations

Because the environment can change, adaptive evolution is a continuous process

Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment

Page 63: Chapter 23: The Evolution of Populations

Sexual Selection

Sexual selection is natural selection for mating success

It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

Page 64: Chapter 23: The Evolution of Populations

Fig. 23-15

Page 65: Chapter 23: The Evolution of Populations

Intrasexual selection is competition among individuals of one sex (often males) for mates of the opposite sex

Intersexual selection, often called mate choice, occurs when individuals of one sex (usually females) are choosy in selecting their mates

Male showiness due to mate choice can increase a male’s chances of attracting a female, while decreasing his chances of survival

Page 66: Chapter 23: The Evolution of Populations

How do female preferences evolve? The good genes hypothesis suggests that

if a trait is related to male health, both the male trait and female preference for that trait should be selected for

Page 67: Chapter 23: The Evolution of Populations

Fig. 23-16

SC male graytree frog

Female graytree frog LC male

graytree frog

EXPERIMENT

SC sperm Eggs LC sperm

Offspring ofLC father

Offspring ofSC father

Fitness of these half-sibling offspring compared

RESULTS

1995Fitness Measure

1996

Larval growth

Larval survival

Time to metamorphosis

LC better

NSD

LC better(shorter)

LC better(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.

Page 68: Chapter 23: The Evolution of Populations

Fig. 23-16a

SC male graytree frog

Female graytree frog LC male

graytree frogSC sperm Eggs LC

sperm

Offspring ofLC father

Offspring ofSC father

Fitness of these half-sibling offspring compared

EXPERIMENT

Page 69: Chapter 23: The Evolution of Populations

Fig. 23-16b

RESULTS

1995Fitness Measure 1996

Larval growth

Larval survival

Time to metamorphosis

LC better

NSD

LC better(shorter)

LC better(shorter)

NSD

LC better

NSD = no significant difference; LC better = offspring of LC malessuperior to offspring of SC males.

Page 70: Chapter 23: The Evolution of Populations

The Preservation of Genetic Variation

Various mechanisms help to preserve genetic variation in a population

Page 71: Chapter 23: The Evolution of Populations

Diploidy

Diploidy maintains genetic variation in the form of hidden recessive alleles

Page 72: Chapter 23: The Evolution of Populations

Balancing Selection

Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

Page 73: Chapter 23: The Evolution of Populations

Heterozygote Advantage

• Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes

• Natural selection will tend to maintain two or more alleles at that locus

• The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

Page 74: Chapter 23: The Evolution of Populations

Fig. 23-17

0–2.5%

Distribution ofmalaria caused byPlasmodium falciparum(a parasitic unicellular eukaryote)

Frequencies of thesickle-cell allele

2.5–5.0%7.5–10.0%

5.0–7.5%

>12.5%10.0–12.5%

Page 75: Chapter 23: The Evolution of Populations

Frequency-Dependent Selection

• In frequency-dependent selection, the fitness of a phenotype declines if it becomes too common in the population

• Selection can favor whichever phenotype is less common in a population

Page 76: Chapter 23: The Evolution of Populations

Fig. 23-18

“Right-mouthed”

1981

“Left-mouthed”

Freq

uenc

y of

“lef

t-m

outh

ed”

indi

vidu

als

Sample year

1.0

0.5

0’82

’83

’84

’85

’86

’87

’88

’89

’90

Page 77: Chapter 23: The Evolution of Populations

Neutral Variation

Neutral variation is genetic variation that appears to confer no selective advantage or disadvantage

For example, Variation in noncoding regions of DNA Variation in proteins that have little effect on

protein function or reproductive fitness

Page 78: Chapter 23: The Evolution of Populations

Why Natural Selection Cannot Fashion Perfect

Organisms1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the

environment interact